An absolute distance metrology is a method of measuring a desired distance at once without the motion of a measurement target. Currently, widely used length measuring instruments based on interference principles measure the relative displacement between an initial position and an end position of the measurement target by continuously integrating a phase variation of an interference pattern caused by the motion of the measurement target.
This method is a measurement method to be traceable to a length standard with high measurement uncertainty since an optical frequency comb of a femtosecond pulse laser can be used as a ruler. This method has an advantage in that measurement may be rapidly executed without a complicated calculation process.
However, the measurement target has to be moved as far as you want when measuring a distance due to phase ambiguity. Since the phase variation is continuously integrated, various error components during the measurement are also accumulated.
Hence, a necessity of a measuring instrument capable of measuring an absolute distance while making use of various advantages of a conventional interferometer has been proposed and researches and developments have been also actively carried out.
The absolute distance metrology based on the interference principles includes various methods including a multi-wavelength interferometer which measures the same distance using two or more optical wavelengths, a frequency sweeping interferometer which measures a distance by sweeping an optical frequency in predetermined profile.
Among them, the multi-wavelength interferometer is one of the absolute distance metrological techniques capable of measuring the distance with the high measurement uncertainty and making use of the advantage of an optical interferometer using a short wavelength of the light, but the multi-wavelength interferometer has a demerit in that a distance to be measured should be estimated within a predetermined range, whereby the multi-wavelength interferometer is used only in a specific field such as gauge block calibration in which the distance to be measured may be initially estimated.
The previous absolute distance interferometry could not achieve measurement uncertainty of a level which can be obtained in a relative displacement measurement due to technical problems such as the stability of the wavelengths in use, an algorithm error, and the like.